Extending battery life with soft-start circuit

This article examines a characteristic found in many battery-powered products. When a circuit is first powered, a high current spike occurs while it establishes stable operating conditions. While a new or recently charged battery can perform reliably and as expected, a partially charged battery cannot support high startup currents (also known as inrush current). As a battery drains, its internal resistance increases, causing a larger voltage drop under load. The larger voltage drop caused by the startup current spike triggers the power supply's undervoltage shutdown mechanism.

During the shutdown process, the battery voltage recovers, causing the power supply to start up again and generate a high surge, which in turn triggers another brownout condition. As a result, the device cycles back and forth between the ON and OFF states, causing user dissatisfaction and forcing the user to replace or recharge the battery prematurely.

Battery life is an important factor in users' purchasing decisions, so any mechanism that can influence the expected battery life will add value to the end product.

把软启动定为例行程序，让电流在一段受到控制的时间内提升到系统要求的数值，可以避免由涌浪电流造成的问题。例行软启动的操作有两大优点。首先，它使输出电压不会上升得太快，能够防止输出电压过冲或显著降低过冲的幅度。

Second, it prevents the voltage drop that accompanies a large inrush current from a partially discharged battery. Routine soft-start reduces the magnitude of the inrush current and therefore the voltage drop that occurs at startup, keeping the battery voltage above the threshold that would trigger a system undervoltage shutdown.

For some types of devices, soft start can be easily implemented. For example, low dropout (LDO) regulators can easily implement soft start and inrush current protection by using gate control. For DC-DC converters, the implementation method depends on whether the device is a buck or boost regulator.

Buck converters and some boost converters do not have a startup block. If the worst case scenario, when the input voltage is at its lowest, is still sufficient to supply voltage to all the blocks of the chip, then it is easy to deal with, because the system can limit the current by means of a maximum current detector, and this limitation will be released after a fixed period of time or when a set of output voltages reaches a predetermined value. This is a very attractive feature because the chip can remain in normal operating mode.

However, some boost converters include a startup block. When these systems operate in boost mode, the input voltage is lower than the voltage requirement, so the output of the converter is used to power the converter. In this case, implementing a soft start becomes complicated because the routine must limit the current throughout the startup process until the output reaches the required output voltage. This means that the soft start mechanism must operate on both the battery supply and the converter output supply.

At the same time, standard linear chips are designed for a wide range of applications and can be used with a variety of related parts, which makes the deployment of soft start more complicated. This also means that the time required to reach the output requirement varies in different applications. Therefore, it becomes extremely challenging to provide a universal soft start solution for a variety of products and applications.

现在，随着奥地利微电子公司推出了一种崭新的器件，在一个产品平台上实施软启动便大为简化。AS1344不仅为絶大部分应用提供完整的启动解决方案，亦带来一系列启动期间的最大电流选择。这让系统可以调节达致所需输出电压的时间。

Figure 1 shows the block diagram of the AS1344. Both PMOS switches are in the OFF state during the shutdown period (the output is disconnected from the input to stop current flow during the shutdown period); both remain in the ON state in normal operation mode.

However, during startup, only one PMOS switch (the one connected to VIN) is in the ON state. Therefore, the designer can adjust the maximum current allowed to pass through the system during startup by changing the value of the external resistor connected between the battery and VIN. This resistor will permanently limit the current flowing to VIN.

Shortly after the DC/DC converter reaches the required output level, the switch from VDD to SW is enabled, in other words, the device is in normal operation mode. This allows more current to pass to support higher load currents, resulting in a more efficient system.

Figures 2 and 3 illustrate these effects. Figure 2 shows that if the input voltage is 1.8V, the design can limit the inrush current to about 200mA through a 3Ω external resistor. Figure 3 shows that when the input voltage is 2.4V and the resistor value is the same as 3Ω, the maximum inrush current value will be about 400mA.

From Table 1, we can see the maximum inrush current values ​​and startup times caused by different external resistors (0Ω to 3Ω) at the same 2.4V input voltage. This table clearly shows that if the system has no external resistor (i.e. Rv=0Ω), it can start quickly and pass a large inrush current.

Table 1: Maximum inrush current and startup time for different external resistors (0Ω to 3Ω) at 2.4V input voltage.

Designers can easily control inrush current by selecting a single external resistor value using the AS1344. By using this single boost converter across different circuit designs, OEMs have the opportunity to reduce procurement costs while simplifying supply chain and warehousing requirements.